The Bergen proton CT system

The Bergen proton Computed Tomography (pCT) is a prototype detector under construction. It aims to have the capability to track and measure ions’ energy deposition to minimize uncertainty in proton treatment planning. It is a high granularity digital tracking calorimeter, where the first two layers will act as tracking layers to obtain positional information of the incoming particle. The remainder of the detector will act as a calorimeter. Beam tests have been performed with multiple beams. These tests have shown that the ALPIDE chip sensor can measure the deposited energy, making it possible for the sensors to distinguish between the tracks in the Digital Tracking Calorimeter (DTC)

Characterization of monolithic CMOS pixel sensor chip with ion beams for application in particle computed tomography

Particle computed tomography (pCT) is an emerging imaging modality that promises to reduce range uncertainty in particle therapy. The Bergen pCT collaboration aims to develop a novel pCT prototype based on the ALPIDE monolithic CMOS sensor. The planned prototype consist of two tracking planes forming a rear tracker and Digital Tracking Calorimeter (DTC). The DTC will be made of a 41 layer ALPIDE-aluminum sandwich structure. To enable data acquisition at clinical particle rates, a large multiplicity of particles will be measured using the highly-granular ALPIDE sensor. In this work, a first characterization of the ALPIDE sensor performance in ion beams is conducted. Particle hits in the ALPIDE sensor result in charge clusters whose size is related to the chip response and the particle energy deposit. Firstly, measurements in a 10 MeV 4He micro beam have been conducted at the SIRIUS microprobe facility of ANSTO to investigate the dependence of the cluster size on the beam position over the ALPIDE pixel. Here, a variation in cluster size depending on the impinging point of the beam was observed. Additional beam tests were conducted at the Heidelberg Ion-Beam Therapy Center (HIT) investigating the cluster size as a function of the deposited energy by protons and 4He ions in the sensitive volume of the ALPIDE. Results show the expected increase in cluster sizes with deposited energy and a clear difference in cluster sizes for protons and 4He ions. As a conclusion, the variation in cluster size with the impinging point of the beam has to be accounted for to enable accurate energy loss reconstruction with the ALPIDE. This does, however, not affect the tracking of particles through the final prototype, as for that only the center-of-mass of the cluster is relevant

Characterization of monolithic CMOS pixel sensor chip with ion beams for application in particle computed tomography

Particle computed tomography (pCT) is an emerging imaging modality that promises to reduce range uncertainty in particle therapy. The Bergen pCT collaboration aims to develop a novel pCT prototype based on the ALPIDE monolithic CMOS sensor. The planned prototype consist of two tracking planes forming a rear tracker and Digital Tracking Calorimeter (DTC). The DTC will be made of a 41 layer ALPIDE-aluminum sandwich structure. To enable data acquisition at clinical particle rates, a large multiplicity of particles will be measured using the highly-granular ALPIDE sensor. In this work, a first characterization of the ALPIDE sensor performance in ion beams is conducted. Particle hits in the ALPIDE sensor result in charge clusters whose size is related to the chip response and the particle energy deposit. Firstly, measurements in a 10 MeV $^{4}$ He micro beam have been conducted at the SIRIUS microprobe facility of ANSTO to investigate the dependence of the cluster size on the beam position over the ALPIDE pixel. Here, a variation in cluster size depending on the impinging point of the beam was observed. Additional beam tests were conducted at the Heidelberg Ion-Beam Therapy Center (HIT) investigating the cluster size as a function of the deposited energy by protons and 4 He ions in the sensitive volume of the ALPIDE. Results show the expected increase in cluster sizes with deposited energy and a clear difference in cluster sizes for protons and 4 He ions. As a conclusion, the variation in cluster size with the impinging point of the beam has to be accounted for to enable accurate energy loss reconstruction with the ALPIDE. This does, however, not affect the tracking of particles through the final prototype, as for that only the center-of-mass of the cluster is relevant

Measurement of beauty and charm production in pp collisions at √s = 5.02 TeV via non-prompt and prompt D mesons

The pT-differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity (|y| < 0.5) in proton-proton collisions at s√s = 5.02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19.3 ± 0.4) nb−1. D mesons were reconstructed from their decays D0 → K−π+, D+ → K−π+π+, and D+s→φπ+→K−K+π+Ds+→φπ+→K−K+π+ and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D+ and D+sDs+ mesons have an extended pT coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on pT, allowing for a more precise determination of their pT-integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons, fs/(fu + fd), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The bb¯¯¯bb¯ production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is dσbb¯¯¯/dy∣∣|y|<0.5=34.5±2.4(stat)+4.7−2.9(tot.syst)dσbb¯/dy||y|<0.5=34.5±2.4(stat)−2.9+4.7(tot.syst) μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section pre- dicted by perturbative QCD calculations

Measurement of beauty and charm production in pp collisions at √s = 5.02 TeV via non-prompt and prompt D mesons

The pT-differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity (|y| < 0.5) in proton-proton collisions at √s = 5.02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19.3 ± 0.4) nb−1. D mesons were reconstructed from their decays D0 → K−π+, D+ → K−π+π+, and D+s→φπ+→K−K+π+ and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D+ and D+s mesons have an extended pT coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on pT, allowing for a more precise determination of their pT-integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons, fs/(fu + fd), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The bb¯ production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is dσbb¯/dy∣|y|<0.5=34.5±2.4(stat)+4.7−2.9(tot.syst) μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section predicted by perturbative QCD calculations

First measurement of coherent ρ0 photoproduction in ultra-peripheral Xe–Xe collisions at √sNN = 5.44 TeV

The first measurement of the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Xe–Xe collisions at sNN=5.44 TeV is presented. This result, together with previous HERA γp data and γ–Pb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+Xe→ρ0+Xe+Xe process, measured at midrapidity through the decay channel ρ0→π+π−, is found to be dσ/dy=131.5±5.6(stat.)−16.9+17.5(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent ρ0 photoproduction at a centre-of-mass energy per nucleon of the γA system of WγA,n=65 GeV is found to be consistent with a power-law behaviour σ(γA→ρ0A)∝Aα with a slope α=0.96±0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit

First measurement of coherent ρ0 photoproduction in ultra-peripheral Xe–Xe collisions at √sNN = 5.44 TeV

The first measurement of the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Xe– Xe collisions at √sNN = 5.44 TeV is presented. This result, together with previous HERA γ p data and γ –Pb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe + Xe → ρ0 + Xe + Xe process, measured at midrapidity through the decay channel ρ0 → π+π−, is found to be dσ/dy = 131.5 ± 5.6(stat.)+17.5 −16.9(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent ρ0 photoproduction at a centre-of-mass energy per nucleon of the γ A system of Wγ A,n = 65 GeV is found to be consistent with a power-law behaviour σ(γ A → ρ0 A) ∝ Aα with a slope α = 0.96 ± 0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit

First measurement of quarkonium polarization in nuclear collisions at the LHC

The polarization of inclusive J/ψ and Υ(1S) produced in Pb–Pb collisions at √sNN = 5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5 < y < 4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters λθ , λφ and λθ φ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2 < pT < 10 GeV/c and pT < 15 GeV/c for the J/ψ and Υ(1S), respectively. The polarization parameters for the J/ψ are found to be compatible with zero, within a maximum of about two standard deviations at low pT, for both reference frames and over the whole pT range. The values are compared with the corresponding results obtained for pp collisions at √s = 7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for Υ(1S) production in Pb–Pb collisions are also consistent with zero

Measurement of beauty and charm production in pp collisions at √s = 5.02 TeV via non-prompt and prompt D mesons

The pT-differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity (|y| < 0.5) in proton-proton collisions at √s = 5.02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19.3 ± 0.4) nb−1. D mesons were reconstructed from their decays D0 → K−π+, D+ → K−π+π+, and D+s→φπ+→K−K+π+ and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D+ and D+s mesons have an extended pT coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on pT, allowing for a more precise determination of their pT-integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons, fs/(fu + fd), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The bb¯ production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is dσbb¯/dy∣|y|<0.5=34.5±2.4(stat)+4.7−2.9(tot.syst) μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section predicted by perturbative QCD calculations

Λc+ Production and Baryon-to-Meson Ratios in pp and p-Pb Collisions at √sNN=5.02 TeV at the LHC

The prompt production of the charm baryon Λ+c and the Λ+c/D0 production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at √sNN=5.02 TeV. These new measurements show a clear decrease of the Λ+c/D0 ratio with increasing transverse momentum (pT) in both collision systems in the range 2<pT<12 GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/π and Λ/K0S. At low pT, predictions that include additional color-reconnection mechanisms beyond the leading-color approximation, assume the existence of additional higher-mass charm-baryon states, or include hadronization via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in e+e− and e−p collisions significantly underestimate the data. The results presented in this Letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies.publishedVersio